The Kayran: Ostmurk

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Ostmurk is a mold that only grows in the darkest areas. This quest is a side quest to the Kayran. Geralt is advised by Triss to gather ostmurk, which is a key ingredient to the Mongoose Potion. Drinking this potion grants +20000% Poison Resistance for 10 minutes, which renders Geralt immune to any and all poison. Simply follow the quest marker to the caves after you talk to Cedric, who can usually be found in the tree stand, just outside the city gates. The cave is located behind the waterfall in the southern part of the forest. Be warned, the cave is heavily infested with nekkers, so drinking Cat potions and Swallow is recommended on normal and higher difficulty. The ostmurk itself is guarded by a greater rotfiend. Once you gather ostmurk and the other ingredients, you can craft the potion.

The kayran's mucus was not only poisonous, but also magical, a fact Triss Merigold confirmed beyond all doubt. A herb known as ostmurk was necessary to prepare the antidote. The plant, as is widely known, grows underground in very damp place. Geralt considered whether to search for such a place himself, or to ask Cedric for help.

If Geralt doesn't stumble on the ostmurk first:

Having in the past worn his boots down on long treks in pursuit of lousy rewards, Geralt decided to give his feet some rest and chose the second option. He went to see Cedric and asked him about ostmurk. As luck would have it, the elf sent him on another trek - deep into the forest, to a cave beneath some elven ruins.

Geralt found some ostmurk in a damp cave beneath the elven ruins and used the herb to prepare a potion that would protect him from the kayran's lethal venom.

The potion was safely hidden in the witcher's bag in anticipation of the fight with the kayran.

I must admit that the potion worked exceptionally well in the fight with the huge beast. The witcher would probably best the kayran even without the protection afforded by the brew, but after all he was a professional. He valued his life too much to expose himself to unnecessary.